1. Field of the Invention
The present invention relates to a display control apparatus for a fluorescent display tube to be utilized for display. More particularly, the present invention relates to a display control apparatus for a fluorescent display tube suited to be combined with electronic apparatus provided with a fluorescent display tube such as microwave ovens and the like.
2. Description of the Prior Art
Fluorescent display tubes are widely utilized in various electronic and electric apparatus. In their utilization of interest to the present invention, such fluorescent display tubes are controlled so that operating procedure or operating state and other data are displayed when the apparatus is in operation, and the time, for example, is displayed when the apparatus is not operated.
A structure of such a fluorescent display tube is described for example in the U.S. Pat. No. 3,723,789. The present invention is directed to a display control apparatus for controlling the above described well known fluorescent display tube in a desired state.
A prior art display control apparatus of interest is described in the following by taking an example of a microwave oven controlled by a microcomputer.
FIG. 1 is a circuit diagram of a fluorescent display tube 1 utilized in a microwave oven. FIG. 2 is a front view showing the arrangement of the display segments of the fluorescent display tube 1.
Referring to FIGS. 1 and 2, the fluorescent display tube 1 comprises a colon segment luminescent portion 2 and four numeral luminescent portions 3, 4, 5 and 6, each portion being coated with a fluorescent material. The numeral luminescent portions 3 to 6 each comprises seven segments 7a, 7b, 7c, 7d, 7e, 7f and 7g. The colon segment luminescent portion 2 and the four numeral luminescent portions 3 to 6 are structured as anodes of the fluorescent display tube 1. The fluorescent display tube 1 further comprises grids 8, 9, 10, 11 and 12 corresponding to the luminescent portions 2 to 6 respectively and one filament 13. In the fluorescent display tube 1 thus structured, a predetermined voltage is applied selectively between the grids 8 to 12 and the colon segment luminescent portion 2 and the seven segments 7a to 7g of the numeral luminescent portions 3 to 6, respectively, so that electrons emitted from the filament 13 are applied selectively to the colon segment luminescent portion 2 and respective seven segments 7a to 7g of the numeral luminescent portions 3 to 6. Thus, desired data is displayed by emission of fluorescent light through the luminescent portions 2 to 6.
In the case of the fluorescent display tube 1 provided in a microwave oven controlled by a microcomputer, the present time is generally displayed when cooking operation is not performed, and for cooking operation, the cooking time set by a user and the remaining time after the start of the cooking operation, the output of the microwave oven as well as other necessary data set by the user such as the maximum cooking temperature and the like are suitably displayed.
In such a case, the present time is displayed by the expression for twelve hours. More specifically, the display in the fluorescent display tube 1 changes successively for each minute from the state of "1:00" representing just one o'clock as shown in FIG. 3(a) to the state of "12:59" representing fifty-nine minutes past twelve o'clock as shown in FIG. 3(b).
If the cooking time is for example just five minutes, a display of "5:00" as shown in FIG. 4(a) is given to the fluorescent display tube 1 and after the cooking is started, the display in the fluorescent display tube 1 changes successively for each second to "4:59", then to "4:58" as shown in FIGS. 4(b) and 4(c) and the cooking is completed with the display of "0".
The display of the output of the microwave oven is given as an arbitrary percentage set by the user. For example, as shown in FIG. 5, a display of "30" is given to represent 30%.
Now, for the purpose of clarifying the problems the present invention intends to solve, detailed description will be made of the light emitting frequency of the respective segments 7a to 7g in the numeral luminescent portion 3 of the most significant digit in the above described fluorescent display tube 1.
For the display of the present time, the numeral luminescent portion 3 of the most significant digit emits light only in the period from ten o'clock to fifty-nine minutes past twelve and in this period, only the segments 7f and 7g representing the numeral "1" emit light.
As for the display of the set cooking time, the cooking time of an electronic cooking range is generally several minutes. In this case, the set cooking time and the remaining cooking time can be displayed only by means of the numeral luminescent portions 4, 5 and 6 of the less significant three digits as shown in FIGS. 4(a) to 4(c), and the numeral luminescent portion 3 of the most significant digit is generally not used. However, in a very rare case, cooking operation is performed for as long as several tens of minutes and in such a case, a display of "24:30" or "35:40" is given to represent the remaining time of 24 minutes 30 seconds or 35 minutes 40 seconds, for example, as shown in FIG. 6(a) and or 6(b), and the numeral luminescent portion 3 of the most significant digit is also used.
For the display of the cooking output, the user can set the output value from 0 to 100% in an arbitrary manner and the set value is displayed by means of the numeral luminescent portions 4, 5 and 6 of the less significant three digits in the fluorescent display tube 1. Accordingly, in this case, the numeral luminescent portion 3 of the most significant digit is not used at all.
As is understood from the above description, although the numeral luminescent portion 3 of the most significant digit, and more particularly the segments 7f and 7g thereof occasionally emit light for the display of the time, the other segments 7a to 7e of the luminescent portion 3 seldom or very rarely emit light.
However, consumers complain that they cannot easily read the displayed data in such segments as the segments 7a to 7e of the luminescent portion 3 with low frequency of light emission where light is very rarely emitted, because the brightness of the light emitted by the segments 7a to 7e of the luminescent portion 3 is lowered as compared with the brightness of the light in the other segments 7f and 7g of the luminescent 7g of the luminescent portions 4, 5 and 6 of the less significant three digits.
As a result of investigation, the causes thereof can be explained as follows. In the fluorescent display tube 1, remaining gas exits. When electrons are emitted from the filament 13 of the fluorescent display tube 1, barium gas and the like are generated from the filament 13. In addition, gas containing suspended matter is generated by collision of the emitted electrons from the filament 13 with the respective luminescent portions 2 to 6 and the grids 8 to 12. These gases tend to accumulate one after another in the luminescent material of the colon segment luminescent portion 2 as well as in the luminescent material of the numeral luminescent portions 3 to 6 as the time passes.
However, if the segments to which the gases move to accumulate emit light occasionally, electrons are applied from the filament 13 to such segments each time the segments emit light. Accordingly, in such segments occasionally emitting light, the gases moved thereto are scattered each time and accumulation of gases in such segments hardly occurs.
On the other hand, electrons are rarely applied to the segments emitting light with extremely low frequency such as the above described specified segments 7a to 7e of the numeral luminescent portion 3 of the most significant digit. Accordingly, in these segments, accumulation of gases proceeds rapidly.
The segments 7a to 7e of the luminescent portion 3 containing thus accumulated gases have an extremely lowered brightness as compared with the other segments. As a result, if data, for example, the remaining time in a long period of cooking is displayed using both the segments not containing accumulated gases and the segments containing accumulating gases, a considerably large difference is caused in the luminescent brightness in the segments, and the display becomes extremely hard to see.
For this reason, in case of using the fluorescent display tube 1 for display of data, all the segments could be made to emit light forcedly for example, for each predetermined interval. In consequence, the above described accumulation of gases would not occur and there would not be a difference in the luminescent brightness in the segments. However, in such a case, there are new problems that the user may often regard the display device as malfunctioning or he may often think erroneous data to be correct by seeing the display forcedly made.